The present invention relates to a three-port circulator with shape that resembles a fork, based on a two-dimensional photonic crystal with triangular lattice.
In optical communication systems, this device can be mostly used on the protection of signal sources and amplifiers against parasitic reflections that can arise in these systems (isolation function).
By employing photonic crystal technology in its design, the presented device can be built with reduced dimensions when compared to circulators based on other technologies. This feature favors the use of the proposed circulator in systems requiring high integration density of components.
Photonic crystals are structures comprised by materials with different refractive indexes, periodically distributed in one, two or three spatial dimensions. They possess a forbidden frequency range, known as photonic band gap, whose existence is associated to the spatial periodicity of these crystals.
Electromagnetic waves with frequency located inside this range cannot propagate along the photonic crystal, being totally reflected by it. The functioning principle of most of the devices based on the photonic crystal technology is associated to the existence of the photonic band gap.
Circulators are mainly used to perform the isolation function in integrated communication systems. This function refers to the protection of signal sources and amplifiers against parasitic reflections, usually coming from unmatched components connected to these systems. These reflections cause instabilities on the functioning of these systems and their effects can be mitigated by the utilization of devices like the presented circulator.
Because it is a nonreciprocal device, the scattering matrix of a circulator is not symmetrical, that is, the device behavior does not remain the same when the input becomes the output and vice versa. For example, in a three-port circulator (1, 2 and 3), the signals transmission (input port→output port) can occur in the clockwise direction (1→2, 2→3 e 3→1) or in the counterclockwise direction (1→3, 3→2 e 2→1).
Several patents related to photonic crystals based circulators have been deposited, among which we highlight the following ones.
The circulator described in the patent of invention US20120243844 is based on a two-dimensional photonic crystal comprised by a triangular lattice of holes etched in a material with magneto-optical properties, subjected to an external magnetic field. This circulator possesses three waveguides (three ports) symmetrically connected to a resonant cavity located in the center of the device, and its format resembles the letter Y.
On the other hand, the report describing the patent of invention US20130223805 presents a circulator based on a two-dimensional photonic crystal comprised by a square lattice of dielectric rods immersed in air. It possesses three waveguides connected in a T-shape and a resonant cavity comprised by ferrite rods (whose magnetic permeability varies accordingly to the intensity of an external magnetic field) and dielectric rods with different radius (when compared to the remaining rods that comprise the photonic crystal).
The patent of invention US20120251048 refers to a four-port circulator based on a two-dimensional photonic crystal with triangular lattice of holes etched in a magneto-optical material. In this case, two coupled resonant cavities are inserted in the photonic crystal. The properties of the magneto-optical material in which these cavities are based depend on the intensity of an external magnetic field applied to the material.
Finally, the circulator described in the patent CN104101947 possesses three ports and it is based on a two-dimensional photonic crystal with square lattice of dielectric rods (with square cross-section) immersed in air. The resonant cavity that comprises the device is based on a central dielectric rod with square cross-section, made of a magneto-optical material, and on four dielectric rods with triangular cross-section, while the three waveguides are connected to the cavity so that the final layout resembles the format of the letter T.